Many devices used in commerce, such as mass spectrometers for material analysis and biotechnology analysis, involve ion beams. When working with ion beams in high-vacuum setups, several standard ion-optical elements are widely used to direct, store and focus ion beams. Among the standard ion-optical elements are quadrupole deflectors, cylindrical deflectors and spherical deflectors. These deflectors, among other ion optical elements are described in the book Building Scientific Apparatus: A Practical Guide to Design and Construction (Third Edition)—John H. Moore, Christopher C. Davis, and Michael A. Coplan, Perseus Books, Cambridge, Mass., 2003, ISBN 0-201-13187-0.
Cylindrical deflectors are easily built and operated. They use electrodes that consist of two curved metallic plates that form corresponding portions of concentric cylinders and are planted inside a vacuum chamber. A positive voltage +V is applied to one of the plates; and, the same voltage with opposite sign −V is applied to the other plate. A charged atom or molecule of the right kinetic energy entering the space between the plates and parallel to the plates at an entry port or aperture will follow the curvature of the plates and thus be guided along a defined beam arc to exit at an arbitrary deflection angle from its direction at the entry port. Other charged or uncharged atoms or molecules will not follow the defined arc and will be lost from the beam that exits the deflector at an exit port or aperture. The properties of the cylindrical deflector are such that it will focus an incoming parallel ion beam in the plane of the deflection while it does not have any influence on the momentum of the ions in the direction perpendicular to that plane. That means an incoming beam with a round cross section (a “round beam”) will leave the deflector as a deflected beam with an oval cross section (an “oval beam”). In order to rectify the deformation of the cross section of the ion beam, asymmetrical lenses or other ion optical elements are used in concert with the cylindrical deflector.
A quadrupole deflector (called simply a quadrupole) consists of four parallel rods, forming two pairs of opposing rods, perpendicular to the plane of deflection. By applying appropriate voltages to these pairs of opposing rods, an incoming ion beam can be deflected by an arbitrary angle. Like the cylindrical deflector, a quadrupole focuses in the plane of deflection while it leaves the beam unfocused in the perpendicular direction. Hence, usually quadrupoles are also used with corrective electrodes or ion lenses along with additional power supplies at the entrance and exit to correct for the distortion of the transmitted beam cross section shape. Much higher voltages (e.g., +/−9800 volts) are required in quadrupoles compared to cylindrical deflectors (e.g., +/−2300 volts) to deflect the same ion beam (e.g., having an energy of 10 thousand electron Volts, keV) by an angle of 90 degrees.
Spherical deflectors are more complicated to build. They use electrodes that consist of portions of two nested spheres with different radii. Again, two voltages of opposite sign are applied and ions can be guided between the electrodes. Unlike the cylindrical deflector or the quadrupole, a spherical deflector creates a focus in both directions. That means a beam entering the structure with a round cross section will retain a round cross section upon exiting the deflector.